But if you want to talk significant plot points, you could just as easily compare the death of the emperor in Oblivion to Skyrim...which had most of the Dark Brotherhood quest line devoted to it, involved multiple stages of planning and setup, deception, betrayal, near-destruction of the guild, some really horrific scenes, the ultimate confrontation and a satisfying denouement.
And the only dungeon involved is a short duck into one to meet the client. After that you're assassinating someone in broad daylight at a wedding, getting a guy's schedule as he patrols Skyrim and ambushing him wherever convenient, killing a chef within a keep and hunting down the world famous Gourmet at the inn he's staying at, infiltrating the Emperor's dining room, and finally his ship. A strictly urban and personal series of missions.
It's always six of one, half a dozen of the other. Anything one does well, the other has a near equal example of doing well, and often the edge goes to Skyrim.
Dark Brotherhood was good in Skyrim. Yet I can't think of a quest besides the 2 you've mentioned (Wedding one and assassinating the emperor on the ship). But that itself brings another problem. In the emperor quest, you could literally just kill everyone on the ship alongside the emperor, and the reward stays the same if I'm remembering correctly. It can be played as yet another dungeon crawl, yet this time on a ship. Oblivion incentivized stealth by offering unique rewards to completing the mission in unique ways. Besides,the death of the emperor has no frigging effect on the world at all besides a couple of lines here and there from guards. If we're talking significant plot points, then this was actually an example of how terrible the plot is in Skyrim. Skyrim takes place during a civil war, with extremely fragile relationships between the Thalmor and the empire. You literally assassinate the emperor, and yet this changes absolutely NOTHING in the game. Not the civil war questline, not the main plot, not the relationship with the Thalmore, nothing. It's poorly thought out.
But yeah, Dark Brotherhood isn't bad at all in Skyrim. All the other guilds, however, are terrible. Companions was the nearly the worse of the bunch of course, with an opponent faction that might as well be regular bandints (they most definitely were reskinned bandits without a single named character that is relevant. The witches were also just regular hagravens). The thieves guild wasn't bad, but absolutely pales in comparison to Oblivion's since most thieving you'll be doing involves radiant quests that have you sneak into NPC's 2x2 ft houses and stealing a few things. Compare that to Oblivion which has you sneak into a unique location for the majority of the quests. College of Winterhold is absolutely the worst, which is only to be expected considering mages' guild in Oblivion was also the weakest. Yet still, College of Winterhold was far worst, having you go through yet more regular old dungeon crawls with nothing else. Oblivion's at least had you do stuff like creating your own staff, deal with enchanted items that affect you mechanically, use magic to solve puzzles, and deal with the inner politics of the different guild hubs. It wasn't very good, but it made me feel like a member of an actual organization of mages.
Here's Skyrim's college of winterhold questline: Get a quick lesson, which I actually enjoyed since it made me feel like a mage for a few minutes. > Dungeon crawl > dungeon crawl > decent investigative quest > dungeon crawl > decent quest about defending town > dungeon crawl > boss fight and ending
None of the dungeon crawls even involve you doing anything mage-like if your character was not a mage. You could just go in with heavy armor and a 2-handed mace all the same, like any other dungeon in the game. The side quests for the college of winterhold also usually ask you to delve into yet another dungeon to retrieve an item, or go around the college looking for someone's misplaced items and whatnot. There are a few exceptions, but that's the point: the good, interesting stuff is the exception here, not the norm.
I know I'm being very harsh on Skyrim here. Oblivion's guild-quests also usually involve a dungeon crawl. But they usually involve much more story and doing stuff OUTSIDE of dungeons. That's the difference here. All I remember from guild's in Skyrim are dungeon crawls. Thinking about Oblivion, on the other hand, and I remember setting up meeting with counts (who may or may not be vampires), dealing with other guild members who end up betraying you, actually interacting with other factions who end up being the enemy, and more. Some of that happens in Skyrim (see why Dark Brotherhood is actually good in Skyrim), but Oblivion has that stuff in spades.
It's worth mentioning that Skyrim's deadric quests are usually far better than their oblivion counterparts, and I do agree that the world in Skyrim is far more varied and diverse. Forests in Oblivion were randomly generated if I'm not mistaken, and exploring for the sake of exploration wasn't very rewarding due to loading hiccups and repetitive landscapes. I just think that, when speaking in terms of quest design, Obliviion comes out at top in most side quests and guild quests, with the exception of the deadric quests.
On a different note, Dragonborn was a good expansion pack. However, I remember the main questline in it lasting less than 3 hours, and I'm sure I'm not mistaken about that since I went through it again earlier this year. Side quests were fine, but nothing in Dragonborn comes toe-to-toe with Sheogorath and how unique the Mania and Dimensia regions were.
